The present disclosure relates generally to radio receivers and transmitters, and more particularly to multichannel radio frequency identification (RFID) transponders and systems. RFID transponders are typically small devices that may include a battery to power internal circuitry, but often operate using energy that can be harvested from an RF field generated by an RFID reader. Low-power transponders often begin operation with zero internal power and generate internal supply power using energy received through a transponder antenna. In operation, the transponder receives a specific RF signal from a reader, and responds by transmitting an RF signal with specific characteristics that can be detected by the reader. The RF signals generated by the reader and transponder are typically modulated with data, allowing data exchange between the reader and the transponder. This feature of low-power RF transponders is useful in a variety of applications, such as automotive devices. RF transponders include an antenna to receive signals and/or energy from a reader, as well as to transmit data to the reader. The data and power reception is typically dependent upon the relative orientation of the reader antenna and the transponder antenna.
Multichannel (e.g., 3-D) transponders are useful where the relative positioning of the reader and the transponder is variable. For example, automotive immobilizer systems include an RFID reader positioned in a fixed location in a vehicle, and a 3-D transponder is mounted to a key fob. A user may be allowed entry to a vehicle and/or permitted to press a start button to start the vehicle if the vehicle's RFID reader properly detects the key fob transponder. However, the relative position of the fixed RFID reader antenna and the key fob-mounted transponder antennas is indeterminate. Accordingly, such immobilizer systems often use 3-D transponder antenna arrangements where a set of three antennas are mounted at mutually orthogonal orientations on the fob. The initial transmission of data from the reader to the transponder is referred to as downlink communications. Once a transponder receives data from the reader, the transponder replies with transmitted information referred to as uplink communications.
The downlink communications is typically accomplished by a reader device modulating the amplitude of a carrier signal for different data states. For example, the reader can transmit an RF signal at or above a predefined minimum amplitude at a given carrier frequency to represent a first data state (e.g., “1”), and transmit no signal or a signal less than the predefined minimum amplitude at the carrier frequency to represent a second binary state (e.g., “0”). Other possible ASK modulation schemes can be used, for example, to provide three or more discernible data states based on different detectable signal amplitudes. In many applications, a transponder must demodulate an ASK signal with a very wide dynamic range for the input signal at a low power dissipation, particularly where the transponder harvests energy from the received RF signal. One possible approach is to use envelope detection circuitry form demodulation, such as a rectifier and a low-pass filter to provide a signal for threshold comparison. However the envelope can vary significantly for different field strengths and antenna conditions, and the resulting ASK demodulation is often unreliable. Similar envelope detection can be used to select a particular one of multiple (e.g., three) transponder antennas for use in uplink communications.